INTRODUCTION
While last year’s general idea was in the ballpark, we saw an under-performance of an already fairly conservative snowfall forecast, and some of the other forecast details turned out a little different, but the idea of a not-too-harsh winter was certainly on the right track. So now we move to prognosticating pattern potential for winter 2024-2025. Is this the year the “classic New England winter” returns? Or are we in for another version of what we’ve seen the last few winters? Or something in between? Or a “big winter”? These are the questions I attempt to answer in this outlook, so let us go forth and forecast!
OVERVIEW
Where last year’s main player was El Nino, the indications are we’ll be eyeing a weak La Nina this winter following the currently ENSO neutral conditions. For snow lovers, the MJO has been an enemy of you during the last several winters, so we’ll look at that to see if it may play more favorably for at least providing better opportunity for something to happen. The big indices such as PDO, AO, NAO, AMO, PNA will all be considered as usual, and predicted as best as we can see then playing out. And don’t forget about QBO too! I’ll remind you what these all mean as we get to each of them, and the type of influences they typically have. Keep in mind, no single index drives the entire pattern. It’s a combination, and with so many indices in play, the number of combinations is high. The aim here in the seasonal forecast is to try to identify the major players and their expectations, then try to fit that together into a refined pattern outlook. Also, not to be forgotten will be the ongoing, long-lasting Hunga Tonga volcano’s influence on the global patterns due to the immense amount of water vapor still present in the stratosphere, having acted as a greenhouse gas temperature boost over the last 2 1/2 years, and effects likely to continue through and well beyond this coming winter. We also have a significant drought ongoing this autumn, and statistics show that our drier autumns have often been followed by lower than typical snowfall totals in the region. Will this be a factor this season? Well, statistics say it may be.
MAJOR INDICES / IMPACTS
Hunga Tonga Volcanic Eruption, January 2022 (HTE): While we don’t know the full details of the impact of this event, it is theorized that the impacts can last at least a few to several more years, so this plays as a bit of a wild card, but has largely been a warming influence globally, due to the immense amount of moisture in the stratosphere as a result of the eruption. It will take quite some time for that to remove itself from that layer, so it follows that the impacts of having it there cannot be ignored.
El Nino Southern Oscillation (ENSO): The general idea heading toward winter is for a weak east-based La Nina, but this has struggled to get going as expected, as indications were it would emerge, even though weakly, in the autumn. While anomalies are a little on the cool side of average, they are close enough to average to still be considered ENSO neutral. My feeling is that we may see this struggle continue, and we’ll be between neutral and very weak La Nina conditions for the majority of the winter, with maybe a bit more pronounced, albeit still weak, La Nina noted by later in the winter. If this is the case, the correlations as far as ENSO goes would be closer to those anticipated for a neutral winter. Early on, this would allow for more cold to be able to be driven into the eastern US with less ridging in the Southeast and less of a push of the jet stream to the north. If La Nina does gain more identity later in the winter, the Southeast ridge would become more prominent and the subsequent pushing of the jet stream to the north would be more likely, allowing for milder weather more often than colder weather in our area. Again, one factor among many, and not as simple as “yes” or “no” regarding any or all types of weather.
Arctic Oscillation (AO): This is the index that indicates the strength and behavior of the Polar Vortex (PV). I still alternate between being amused and frustrated when I see the term “Polar Vortex” used somewhat out of context on social media and even some mainstream media, as if it’s a “new” phenomenon of some kind or some kind of “polar storm”. It’s been used to describe the onset of severe cold, but also used to describe the opposite. Leaving the drama of the media out of it, there’s actually truth to that. The stability of the PV is a high determining factor on how much cold air is able to move equatorward from the polar regions. A tight, strong PV, limits the movement of cold, while a disturbed, weakened PV allows more to take place. That’s the basic idea. The specifics help determine the “where” and “when”, and of course other aspects of the large scale pattern help determine the behavior of this feature. Currently, we’re looking at an AO that’s about neutral and wants to trend negative as we head through the late November / early December period. This in itself can allow colder weather to build a lot closer to our latitude and potentially make more notable visits as we head into the last weeks of autumn, but again it would be dependent on the specifics of the large scale pattern. A negative AO doesn’t automatically mean that everywhere further south gets colder. There are pockets of cold, and pockets of milder. Sometimes they are stable and don’t move much, other times they are more longitudinally migratory. An example of this would be an initial cold snap in the US West while the East is mild, and then the pattern shifts eastward to chill down the East while the West warms. Just one example – not a specific forecast. As I write this, a slightly negative AO looks like it wants to deliver colder weather to our region as we start meteorological winter, and the question is how long into December does that contribute to cold shots? My current guess is pretty deep into the month. There are some notable indications that the PV will become stronger and therefore the AO will become more stubbornly positive as we get to 2025 (mid to late winter). This would increase the chance of milder weather and limit the amount of cold that can invade, since the jet stream is stronger and displace northward, bottling up the arctic air well to the north. This is at least partially related to the status of the next major index in my discussion. Read on…
Quasi-Biennial Oscillation (QBO), the direction of stratospheric winds above the tropical latitudes: This recently flipped to a positive, or westerly QBO phase. A westerly QBO is often a player in triggering milder winter weather in the eastern US. With the phase having flipped recently and not too strong yet, its influence may be muted at first, before becoming more pronounced.
North Atlantic Oscillation (NAO): In its positive phase, this index describes general low pressure north and high pressure south in the North Atlantic Ocean, and the pattern in this phase also is a fairly progressive jet stream pattern, often blowing across Canada and/or the northern US and into North Atlantic without being hindered along their path. In its negative phase, high pressure is more dominant in the north, with low pressure to the south. This describes a blocking pattern, which with high pressure sitting over the North Atlantic, Greenland, or Atlantic Canada, forces the jet stream to the south. This tends to be associated with colder and/or stormier weather in the US Northeast. However, this can be a dry pattern if the blocking is too strong or oriented in such a way that the high pressure area is too far west. Predicting this index becomes highly uncertain beyond a couple weeks. And over the next couple weeks this index will be floating around neutral with no definitive positive or negative indicated.
Pacific Decadal Oscillation (PDO): a water temperature pattern in the mid latitude Pacific Ocean, north of 20 degrees N latitude. This index went against the El Nino rule of it of being positive and was negative last winter, and it looks like the negative PDO will continue this winter as well. The negative phase of the PDO tends to promote more troughing, colder weather, and storminess in the western US, while a downstream ridge and milder weather occurs in the eastern US.
Pacific-North American Index (PNA), which describes the upper air pattern from the North Pacific into North America): When this index is positive, it’s associated with high pressure ridging in the western portion of North America and a tendency for low pressure troughing in the eastern portion of North America. This winter, like last, the expectation for PNA is opposite, in a negative phase, which tends to aid in more trough development in the western US and more ridiging in the eastern US. This would be a contributing factor for a milder winter in the eastern US.
Eastern Pacific Oscillation (EPO), which is similar to NAO, but in the eastern North Pacific: A positive EPO is a mostly zonal or lower amplitude pattern which allows mild Pacific air into the US more readily, while a negative EPO features more high pressure ridging in the northeastern Pacific and Alaska, blocking Pacific air, and driving colder air southward out of Canada into the US, the longitude of the coldest determined at least in part by the placement of high pressure and downstream troughing. Again, similar to the outlook last year, the EPO is expected to run in a negative phase to start out the winter in December, but with a trend to neutral and then positive as we head through winter. This would put some limitation on Pacific flow. I wrote this last year as well, and it holds true yet again: A negative EPO while the PNA is also negative can result in a dampened impact from both indices, which may be the case to start the winter, rendering them less powerful factors.
Indian Ocean Dipole (IOD), an index that measures water temperature tendencies in the Indian Ocean, and found to have some impact on the large scale climate pattern: A positive IOD features warmer water and increased convective activity in the western Indian Ocean and nearby land masses while cooler water dominates the eastern Indian Ocean with less convective activity and resultant rainfall for southeastern Asia as well as Australia. A negative IOD produces the opposite – cooler water in the western waters, warmer waters to the east with more convective activity and rain in southeastern Asia / Australia. The IOD is expected to be in neutral territory this winter, but close to weak negative in December and closer to weak positive from January onward. While a positive phase is associated with a stronger Polar Vortex, the expectation of neutrality to very weak positive for this phase means its influence will be minimal at best, and a stronger PV would be the result of other factors. So the IOD is not a major factor in this forecast for the 2024-2025 winter.
Madden-Julian Oscillation (MJO), which describes convective focus in equatorial regions and has 8 phases depending on the location of the normally eastward-drifting convective wave: It’s known that the Northeast has a better shot at snow / wintry weather with MJO in phases 7, 8, and 1 early in the season and in phases 8, 1, and 2 as the winter progresses. The MJO has been the “enemy” of the snow lover many times in southern New England during the last several winters, and there are some indications that it may continue this habit this winter. We’ve been in a pattern where the MJO is strongest (moderate strength) while in phases 3 through 5, then will display a weakening trend while passing from phases 6 through 7 and nearly non-existent when phases 8, 1, and 2 occur. This index can be hard to predict at times, but most indicators are that this stronger 3 through 6, weaker 7 through 2 pattern will continue for at least the early part of winter. This is not the best news if you want more snowstorm chances for this area, but that said, it doesn’t mean that it makes it impossible to get some meaningful snow, since the MJO is only one factor of many that influence this. Will MJO “play nicer” for the snow lover later on? We’ll see.
Solar Cycle: This cycle, also known as the solar magnetic activity cycle, sunspot cycle, or Schwabe cycle, is a periodic (about 11 year) change in solar activity as measured by solar storms, or sunspots, on the sun’s surface. There has been a correlation observed that high periods of solar activity tend to correspond to milder winters overall, but this is not a solid correlation and more or a base guideline. A more useful aspect of the ability to measure this is to tell when we may see greater or fewer episodes of the aurora, and the potential for disruptions of satellite communication. Either way, Solar Cycle 25 (or the 25th cycle observed since we clearly identified this cycle in the mid 1800s) has been entering peak stronger than expected. This was exemplified by the multiple episodes of aurora observed since spring of 2024. The peak of solar cycle 25 is now not expected to occur until early in the summer of 2025, so we’re firmly in a strong cycle for this winter. There is some decent evidence pointing to this solar max being one of the reason for our recent mild winters, and it may yet try to turn up the thermostat on this winter as well, unless overcome by other indices with the opposite effect.
This covers the major indices, and now it’s time for the reminder of the uncertainty of long range forecasting. This is somewhat different than the uncertainty of day-to-day forecasting. We still have the unknowns of the HTE to factor in, as well as the surprises that can sometimes occur even when you have a seemingly firm grasp of the statuses and expectations of the more-known major indices. So there are wildcards in the deck, and we will find out how many we are dealt.
Like last year, variability will be the key. We’re not going to be cold front one end to the other, or mild from one end to the other, nor are we likely to be consistently dry or wet. There are enough conflicting indices to know we’ll see a few battles to see which one has the greater impacts, and they may wrestle back and forth a few times, but we still should see some general trends that are easier to spot. I wrote something last year that rings true always regarding seasonal forecasting, so here it is repeated: The aim of the long range forecast is to try to identify what these longer term trends will be. It almost doesn’t matter what the sum of them ends up being, because the impacts of temperature, wind, rain/snow are most strongly felt during the events, not so much over the entire season, but on an event-by-event basis. The longer term trends are more important for things like agriculture, or heating bills. People remember the 2014-2015 winter for its incredible stretch of snow and cold from the very end of January to the very beginning of March, and nearly forget that winter was almost “non existent” up until that point. They remember the individual storms in that stretch, and the stretch itself, but over a much shorter period of time than the entire winter. But let’s still focus on some definitive periods in the monthly breakdowns that follow…
DECEMBER
This month is probably the easiest, at least in theory, to forecast for, since it starts in a matter of hours, and I’m trying to predict trends 31 days out, not 60, 90, or greater. But it is what it is. All the factors discussed above combine to start our December on the cold side, and generally on the dry side too. The big question is, does it stay that way? I suspect at some point we’ll relax the cold shots and get a milder trend or two, but I do think this particular December will continue a set-up to see cold air more easily driven down into the northeastern US while ENSO remains generally neutral and we lack a stronger ridge in the southeastern US. At the same time, an AO that is more neutral and not so strongly positive doesn’t lock the cold away. The jet stream’s orientation says that most of the deeper moisture systems will be driven south of our region, but all it takes is a subtle shift of the pattern to produce a more southwesterly flow and one or more of those systems can come up this way. Do they combine with cold air and give us early season snow, or do they go too far north and keep the snow in the mountains while southern areas are too mild for snow during the storm event? Those details will reveal themselves with time. I do think we’ll have a handful of northern stream systems during our cold pattern that can help us get going with our seasonal snow total in at least a few minor events, so I do not expect a snowless December, even if the larger events to produce well. I’ll go with slightly below normal temperatures and near to below normal precipitation. Snowfall with respect to normal is going to be a tough call since we see the colder start with mainly dry weather, and then some up and down temperatures when the pattern gets a little more active. Inland areas probably stand the best chance of accumulating snow in “larger” systems, but if we get a series of clipper systems in a colder pattern, while the snowfalls are generally lighter they can be more uniform. Based on this I cautiously call for a range of a little below normal east and south to near normal north and west (near to slightly below normal for the region overall).
JANUARY
Looking over the expected behavior of indices described above, this is the month that starts out on the colder, stormier side, followed by a trend toward drier and milder weather later in the month. Temperatures that start cold and end mild average out to near normal, while both precipitation and snowfall are a little above the long term averages.
FEBRUARY
A stronger PV and a positive AO are likely to be the strongest driving forces in the pattern for the 3rd month of meteorological winter. This would indicate a mild month with brief cold shots behind fast-moving storms that track mostly north of our area, and limited precipitation meaning below normal expectations for precipitation and snowfall. Do the weeks prior to this help finish off our ongoing drought? Or is it inadequate only to find the drought fueled further by a dry end to meteorological winter? Well, don’t forget March. Let’s look there.
MARCH (UNTIL EQUINOX)
Weak La Nina, positive AO, negative PNA, negative PDO. These say mild and dry. Wildcard: Maybe a more active Pacific jet aimed a little more our way if we can see some weakening of the PV to help deliver us more precipitation. It’d be a gamble to go this way with a forecast a few months in the future, but I’ll take the leap. Temperatures near to above normal, precipitation a little above normal, snowfall near normal as we wrap up the winter season.
WINTER SEASON OVERALL
Temperature: Slightly above normal (departure +0.5F to +1.5F).
Precipitation: Slightly above normal (departure about +1 inch).
Snow: Near to slightly below normal.
-Boston 40-50 inches
-Worcester 55-65 inches
-Providence 35-45 inches
-Hartford 50-60 inches